Natural Enemies of Aphids

Parasitic wasps

One of the most important groups of natural enemies of pest insects, including aphids, is the parasitic wasps. Parasitic wasps are free-living in the adult stage, but in the larval stage are parasitic on various insects. The parasitic larvae eat their hosts from within, ultimately resulting in the death of the host insect.

Figure 1. Parasitic wasp laying eggs in an aphid Photo credit: Bob Ellingson

B. communis emerging from a parasitized aphid.

Parasitic wasps comprise a very large and diverse group. Their size is dictated by the body size of their host, and therefore they include some of the tiniest of all insects (some actually parasitize the egg stage of other insects), as well as much larger species.
Binodoxys communis, the first aphid parasite to be field tested this summer, is a tiny, stingless wasp smaller than the head of a pin.

Parasitic wasps tend to be fairly specific in what they attack. For example, a species that attacks aphids will not parasitize quite different hosts such as caterpillars or leafhoppers.

It is usually the adult wasp that flies around and finds the appropriate type of host for her offspring. She then lays one or more eggs in or on the host insect (Figure 1). When the eggs hatch, the wasp larvae begin to feed on the host insect and usually stay with that one host until they are fully grown, at which time they pupate. The host insect usually dies at this point.

Parasitized aphids often become inflated as the parasite reaches full size. The next generation wasp will emerge from the host aphid, often by cutting a circular "hatch" in the back of the aphid. When the parasite flies away, just a shell of the host aphid remains; this is referred to as a "mummy" (Figure 2).

Figure 2. An aphid mummy that was parasitized by Binodoxys communis. Photo credit: Dan Mahr

There are nearly always parasitic wasps in the native range of an aphid species. When an invasive aphid species becomes established in a new area, however, there are usually no specialized parasitic wasps within it, which often results in severe outbreaks not normally found in an aphid's native range. Therefore, classical biological control projects attempt to find the aphid's naturally-occurring enemies and introduce them into the pest's new range with the intent of reestablishing a balance similar to that seen in the aphid's natural range.

Parasitic wasps are in the insect order Hymenoptera, which includes the bees, wasps, and ants. There are many families of parasitic wasps. The two most important groups that parasitize aphids include the Braconidae, especially the subfamily Aphidiinae, and the Aphelinidae.